Method of fabricating back-illuminated imaging sensors using a bump bonding technique

ABSTRACT

A method for fabricating a back-illuminated semiconductor imaging device on a semiconductor-on-insulator substrate, and resulting imaging device is disclosed. The method for manufacturing the imaging device includes the steps of providing a substrate comprising an insulator layer, and an epitaxial layer substantially overlying the insulator layer; fabricating at least one imaging component at least partially overlying and extending into the epitaxial layer; forming a plurality of bond pads substantially overlying the epitaxial layer; fabricating a dielectric layer substantially overlying the epitaxial layer and the at least one imaging component; providing a handle wafer; forming a plurality of conductive trenches in the handle wafer; forming a plurality of conductive bumps on a first surface of the handle wafer substantially underlying the conductive trenches; and bonding the plurality of conductive bumps to the plurality of bond pads.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/779,414 filed Jul. 18, 2007, which claims the benefit of U.S.provisional patent application No. 60/908,436 filed Mar. 28, 2007, thedisclosures of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The field of the present invention is semiconductor device fabricationand device structure. More specifically, the present invention relatesto a back illuminated image array device and a method of constructingsuch a device.

BACKGROUND OF THE INVENTION

CMOS or CCD image sensors are of interest in a wide variety of sensingand imaging applications in a wide range of fields including consumer,commercial, industrial, and space electronics. Imagers based on chargecoupled devices (CCDs) are currently the most widely utilized. CCDs areemployed either in front or back illuminated configurations. Frontilluminated CCD imagers are cost effective to manufacture compared toback illuminated CCD imagers such that front illuminated devicesdominate the consumer imaging market. Front side illumination, whiletraditionally utilized in standard imagers, has significant performancelimitations such as low fill factor/low sensitivity. The problem of lowfill factor/low sensitivity is typically due to shadowing caused by thepresence of opaque metal bus lines, and absorption by the arraycircuitry structure formed on the front surface in the pixel region.Thus, the active region of the pixel is typically very small (low fillfactor) in large format (high-resolution) front illuminated imagers.

Thinned, back illuminated, semiconductor imaging devices areadvantageous over front-illuminated imagers for high fill factor, betteroverall efficiency of charge carrier generation and collection, and aresuitable for small pixel arrays. One goal of the performance of backilluminated, semiconductor imaging devices is that the charge carriersgenerated by light or other emanation incident on the backside should bedriven to the front side quickly to avoid any horizontal drift, whichmay smear the image. It is also desirable to minimize the recombinationof the generated carriers before they reach the front side, since suchrecombination reduces overall efficiency and sensitivity of the device.

These desires may be achieved by providing a thin semiconductor layerand a high electric field within this layer. The field should extend tothe back surface, so that the generated carriers, such as electrons orholes, can be driven quickly to the front side. This requires additionaltreatment at the backside of the device, which adds to the complexity ofthe fabrication process. One current technique includes chemicalthinning of semiconductor wafers and deposition of a “flash gate” at thebackside after thinning. This requires critical thickness control of thebackside flash gate. Another technique involves growth of a thin dopantlayer on a wafer back using molecular beam epitaxy (MBE). Still anotherknown method used to provide a desired electric field is to create agradient of doping inside the thinned semiconductor layer by backsideimplant of the layer followed by appropriate heat treatment forannealing and activation. These methods can not be easily included inconventional semiconductor foundry processing, and require moreexpensive custom processing.

Fabrication of thinned back-illuminated imagers has other challenges:For example, thinned back illuminated imagers can have inherent danglingbonds present at the silicon back surface, which may cause generatedelectrons to recombine at the back surface. Therefore, quantumefficiency (QE) can be degraded if the backside of the thinned imager isnot treated to reduce traps. Thinning of wafers poses yield issues suchas stress in the thinned wafer due to non-uniformity of epitaxial layerthickness. For these and the above reasons, fabrication cost is muchhigher for high volume production of back-illuminated imagers than forfront illuminated imagers.

A cost effective process for manufacturing silicon-on-insulator (SOI)based back illuminated CCD/CMOS imagers is proposed in co-pending,commonly owned U.S. patent application Ser. No. 11/350,546, thedisclosure of which is incorporated herein by reference in its entirety.The fabrication method proposed in that application not only solves theabove mentioned problems, but also had several advantages over otherproposals for back illuminated CCD/CMOS imagers, including:

-   -   The proposed method is fully compatible with existing CCD/CMOS        imager foundry processes.    -   The proposed method has no need for any special backside        treatment.    -   The buried oxide layer of the SOI wafers acts as a natural        stopping layer for a high throughput thinning process.    -   The thickness of epitaxial layer grown using this process is        precisely controlled. This, in conjunction with the natural        stopping oxide insulating layer of the SOI, can result in highly        uniform thickness as compared to conventional approaches.    -   The proposed method allows for multi-level metal processing    -   Devices manufactured using the proposed method can be fully        tested before applying the steps of wafer thinning/lamination,        which results in major cost reductions in a production        environment.

Some imaging systems incorporate color filters and micro-lenses into theimage sensors to produce wavelength dependent signals. To date, this hasbeen done mostly with front illuminated imagers. Fabrication of colorfilters and micro-lens for thinned back illuminated imagers, even forthe method proposed above, is a complex process. Alignment of colorfilters/micro-lenses on the backside to the pixels in the front side isvery critical. Back to front alignment is possible, but with less degreeof alignment accuracy. Apart from that, wire bonding and packaging ofsuch back thinned imagers with color filters and micro-lenses add tocomplexity of the process.

Accordingly, what would be desirable, but has not yet been provided, isa device and method for fabricating back illuminated imagers which cancost-effectively incorporate color filters, micro-lenses, and wirebonding techniques.

SUMMARY OF THE INVENTION

Disclosed is a method and resulting device for back-illuminated imagingdevice employing semiconductor-on-insulator (SOI) substrates. The methodfor manufacturing the imaging device includes the steps of providing asubstrate comprising an insulator layer, and an epitaxial layersubstantially overlying the insulator layer; fabricating at least oneimaging component at least partially overlying and extending into theepitaxial layer; forming a plurality of bond pads substantiallyoverlying the epitaxial layer; fabricating a dielectric layersubstantially overlying the epitaxial layer and the at least one imagingcomponent; providing a handle wafer; forming a plurality of conductivetrenches in the handle wafer; forming a plurality of conductive bumps ona first surface of the handle wafer substantially underlying theconductive trenches; and bonding the plurality of conductive bumps tothe plurality of bond pads.

The resulting back-illuminated semiconductor imaging device, comprisesan insulator layer; an epitaxial layer substantially overlying theinsulator layer; at least one imaging component formed at leastpartially overlying and extending into the epitaxial layer; a dielectriclayer formed substantially overlying the at least one imaging componentand epitaxial layer; a plurality of bond pads substantially overlyingthe dielectric layer; and a handle wafer having conductive trenchesformed therein, the conductive trenches being bonded to the plurality ofbond pads.

In some embodiments, at least one imaging component has an imaging area,and wherein at least one of the plurality of bond pads substantiallyoverlying the epitaxial layer also at least partially overlies theimaging area. The plurality of conductive trenches are formed in thehandle wafer by etching vias in the handle wafer; filling the vias witha conducting material; and smoothing a surface of the handle wafer. Thevias are filled with a conductive material using an electroplating orsputtering technique. The conductive material can be a metal such asgold, tin, or wolfram.

A plurality of alignment keys are formed at least partially overlyingand extending into the epitaxial layer. The alignment keys in theepitaxial layer are formed by printing key patterns on a top portion ofthe epitaxial layer; etching the underlying epitaxial layer below thekey patterns using a trench etch process until the etched away siliconis stopped by the underlying insulator/buried oxide layer; and fillingthe opened trenches with one of an oxide of silicon, silicon carbide,silicon nitride, and poly-silicon.

The imaging components can include CMOS imaging components,charge-coupled device (CCD) components, photodiodes, avalanchephotodiodes, or phototransistors, in any combination. The opticalcomponents can include color filters and micro-lenses, in anycombination.

SUMMARY DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a silicon-on-insulator (SOI) substrate employed in aprocess for fabricating a back-illuminated imaging device, according toan embodiment of the present invention;

FIG. 2 shows the step of forming an epitaxial layer on the seed layer ofthe SOI substrate depicted in FIG. 1;

FIG. 3 shows the step of forming alignment keys in the epitaxial layer,according to an embodiment of the present invention;

FIG. 4 shows the step of fabricating one or more imaging components onthe epitaxial layer, and printing and etching bond pad areas in theepitaxial layer at locations between the alignment keys of FIG. 3;

FIG. 5 shows the step of preparing a second handle wafer to be attachedto the side of device containing the one or more imaging components ofFIG. 4;

FIG. 6 shows the step of etching vias in the second handle wafers atlocations that are to be aligned with the bond pad areas of FIG. 4;

FIG. 7 shows the step of filling the vias of FIG. 6 with a conductingmaterial by means of electroplating or sputtering to form filled metaltrenches;

FIG. 8 shows the step of smoothing the surface of and depositing metalon the second side of the second handle wafer;

FIG. 9 shows the step of defining a plurality of bond pads over thefilled metal trenches on the second surface of the second handle wafer;

FIG. 10 depicts the step of forming a plurality of metal bumps on thebond pads on the first side of the second handle wafer;

FIG. 11 depicts the step of bump bonding the second handle waferstructure to the SOI device wafer structure to produce a compositeimager structure;

FIG. 12 shows the step of removing of the first handle wafer belongingto the initial SOI wafer structure of FIG. 1;

FIG. 13 shows the step of flipping the composite imager structure overfor further processing;

FIG. 14 shows the step of bonding optical components to the back side ofthe a composite imager structure using the alignment keys as guides;

FIG. 15 shows the step of fabricating metallic contacts and attachingbond wires to the metallic contacts on the back side of the compositeimager structure;

FIG. 16 is a top plan view of a front illuminated imager, wherein bondpads are distributed about its outer periphery so as to provide anunobstructed imaging area;

FIG. 17A is a top plan view of a back illuminated imager constructed inaccordance with the present invention, wherein bond pads can be placedwithin the pixel area of the front side of the imager, thereby enablingthe reduction of the overall size; and

FIG. 17B is a side view of the back illuminated imager of FIG. 17A,showing the location of vias within the front side imaging area.

DETAILED DESCRIPTION OF THE INVENTION

The following embodiments are intended as exemplary, and not limiting.In keeping with common practice, figures are not necessarily drawn toscale.

FIG. 1-15 illustrate an embodiment of a process for manufacturingthinned back-illuminated imagers and a resulting structure. FIG. 1illustrates initial substrate 10 sometimes referred to in the art as asemiconductor-on-insulator (SOI) substrate. Starting SOI substrate 10,shown in FIG. 1, is composed of handle wafer 25 to provide mechanicalsupport during processing, an insulator layer 20 (which can be, forexample, a buried oxide layer of silicon), and seed layer 15. In thepresent embodiment, the handle wafer 25 may be a standard silicon waferused in fabricating integrated circuits. Alternatively, the handle wafer25 may be any sufficiently rigid substrate composed of a material whichis compatible with the steps of the method disclosed herein. Insulatorlayer 20 may comprise an oxide of silicon with a thickness of about 1micrometer. Among other embodiments, the thickness of insulator layer 20may fall in a range from about 10 nm to about 5000 nm. Seed layer 15 maybe comprised of crystalline silicon having a thickness from about 5nanometers to about 100 nanometers.

SOI substrates are available commercially and are manufactured byvarious known methods. In one method, thermal silicon oxide is grown onsilicon wafers. Two such wafers are joined with oxidized faces incontact and raised to a high temperature. In some variations, anelectric potential difference is applied across the two wafers and theoxides. The effect of these treatments is to cause the oxide layers onthe two wafers to flow into each other, forming a monolithic bondbetween the wafers. Once the bonding is complete, the silicon on oneside is lapped and polished to the desired thickness of seed layer 15,while the silicon on the opposite side of the oxide forms handle wafer25. The oxide forms insulator layer 20.

Another method of fabricating a SOI substrates begins with obtaining amore standard semiconductor-on-insulator (SOI) wafer in which the seedlayer 15 has a thickness in the range from about 100 nm to about 1000nm. A thermal oxide is grown on the semiconductor substrate, using knownmethods. As the oxide layer grows, semiconductor material of thesemiconductor substrate is consumed. Then the oxide layer is selectivelyetched off, leaving a thinned semiconductor substrate having a desiredseed layer thickness.

SOI substrates manufactured by an alternative method, known as SmartCut™., are sold by Soitec, S.A.

Seed layer 15 may comprise silicon (Si), Germanium (Ge), SiGe alloy, aIII-V semiconductor, a II-VI semiconductor, or any other semiconductormaterial suitable for the fabrication of optoelectronic devices.

Referring now to FIG. 2, epitaxial layer 30 is formed on the seed layer15, using seed layer 15 as the template. Depending on the material ofseed layer 15, epitaxial layer 30 may comprise silicon (Si), Germanium(Ge), SiGe alloy, a III-V semiconductor, a II-VI semiconductor, or anyother semiconductor material suitable for the fabrication ofoptoelectronic devices. Epitaxial layer 30 may have a thickness fromabout 1 micrometers to about 50 micrometers. The resistivity of theepitaxial layer 30 can be controlled by controlling the epitaxial growthprocess.

Referring now to FIG. 3, once epitaxial layer 30 is grown, alignmentkeys 45 are printed on and etched into the epitaxial layer 30. Thealignment keys 45 can be used to align subsequent layers during theimager fabrication process and also can be used to align color filterson the backside after the wafers are thinned. The use of alignment keyscan result in highly accurate alignment of about 0.1 micrometer or lessfor subsequently deposited layers. The alignment keys 45 can also beused to open bond pad areas for wire bonding to the backside of theresultant device. Using photolithography, key patterns 50 are printed ona top portion 55 of the epitaxial layer 30. A trench etch process can beused to etch the underlying epitaxial layer 30 below the key patterns 50until the etched away silicon is stopped by the underlyinginsulator/buried oxide layer 20 using plasma etching. The open trenches57 are then filled with a electrically insulating material such as anoxide of silicon, silicon carbide, silicon nitride, or poly-silicon.

Referring now to FIG. 4, one or more imaging components 60 may befabricated on the epitaxial layer 30 using known methods ofsemiconductor fabrication. These imaging components 60 may includecharge-coupled device (CCD) components, CMOS imaging components,photodiodes, avalanche photodiodes, phototransistors, or otheroptoelectronic devices, in any combination. The imaging components 60may include both CCD and CMOS components fabricated in separate areas ofepitaxial layer 30 using known masking methods. Also included may beother electronic components such as CMOS transistors, (not shown)bipolar transistors (not shown), capacitors (not shown), or resistors(not shown). The imaging components 60 may be aligned on the epitaxiallayer 30 using the alignment keys 45 as a guide. When several imagersare fabricated on the epitaxial layer 30, one or more dicing streets 65may be present in the epitaxial layer 30. As a preliminary step in theprocess of attaching a second handle wafer using bump bonds to bediscussed hereinbelow, a dielectric layer 67 made of an oxide or nitrideof silicon may be deposited over the imaging components 60 and thealignment keys 45. A plurality of (conductive) metal bond pads 70 areformed on the dielectric layer 67 overlying the imaging components in apattern which does not obstruct an imaging area 72 of any of the imagingcomponents 60 of what is now a first device structure 73.

Referring now to FIGS. 4 and 5, further processing of components can bemade to the back side 74 of the first device structure 73, necessitatingthe addition of a second handle wafer 75 to the front side 77 of thefirst device structure 73 for providing further mechanical support. Toadd the second handle wafer 75 to the front side 77 of the first devicestructure 73, the second handle wafer 75 can be bump bonded to the bondpads 70 on the front side 77 of the first device structure 73. Thesecond handle wafer 75 can be made of any suitable material thatprovides mechanical support, such as silicon, aluminum nitride, orceramic. A seed layer 80 of any suitable metal, such as gold, tin,wolfram, etc., can be deposited on a first side 85 of the second handlewafer 75. The resulting second device structure 88 is now ready for bumpbonding to the first device structure 73.

Referring now to FIG. 6, a plurality of vias 90 are defined and etchedinto a second side 95 of the second handle wafer 75. The vias 90 arealigned with the positions of the bond pads 70 of the first devicestructure 73. Referring now to FIG. 7, the vias 90 are filled with asuitable conducting material by means of electroplating or sputtering toform filled metal trenches 100. If the second handle wafer 75 is made ofsilicon, electrical isolation needs to be provided between the secondhandle wafer 75 and the filled metal trenches 100. As a byproduct of theelectroplating or sputtering process, excess metal 105 may protrude fromand overhang the metal trenches on the second side 95 of the secondhandle wafer 75. Referring now to FIG. 8, any overhanging metal isremoved and the surface of the second side 95 of the second handle wafer75 is made smooth and flat using a chemical mechanical polishing process(CMP). A metal layer 110 is deposited over the second side 95 of thesecond handle wafer 75 and the filled metal trenches 100. Referring nowto FIG. 9, a plurality of bond pads 115 are defined over the filledmetal trenches 100 using a photolithographic process and by etching awayportions of the metal layer 110 from the second surface 95 of the secondhandle wafer 75. Referring now to FIG. 10, a plurality of metal bumps120 are formed on the bond pads 115 on the first side 85 of the secondhandle wafer 75. The metal bumps 120 are made from a suitable metal suchas indium. The second device structure 88 is now ready for bump bondingto the first device structure 73.

Referring now to FIG. 11, bump bonding of the second device structure 88to the first device structure 73 is depicted. After aligning structures73, 88 using the bond pads 70, 115 as keys, the metal bumps 120 of thesecond handle wafer 75 are bonded to the bond pads 70 on the front side77 of the second device structure 88. Bonding is effected by means ofapplying an appropriate elevated pressure and an appropriate temperatureto the aligned structures 88, 73. Because of the applied pressure andelevated temperature, the metal bumps 120 fuse to the bond pads 70,forming solid metal contacts between the structures 88, 73. To providemechanical support between the structures 88, 73, the resulting gap 130between the structures 88, 73 is filled with a suitable epoxy/glue toform a single imager structure 135.

Referring to FIGS. 12 and 13, the next step of the process include theremoval of the first handle wafer 25. Handle wafer 25 is no longerneeded to provide mechanical stability. Removal of handle wafer 25 maybe accomplished by partial mechanical grinding followed by chemicaletching, mechanical grinding, or a combination of these methods. Withchemical etching, handle wafer 25 may be removed selectively, withoutremoving insulator layer 20, to produce a smooth back side 137 of theimager structure 135. The insulating layer 20 acts as an etch stoplayer. The imager structure 135 is then flipped over (FIG. 13) forfurther processing.

Referring now to FIG. 14, optical components 140 can be bonded to theback side 137 of the imager structure 135 using the alignment keys 45 asprecision guides. The one or more optical components can comprise colorfilters and micro-lenses to produce wavelength dependent signals.Referring now to FIG. 15, if dicing streets 65 are present formanufacturing a plurality of imagers, at this time, the dicing streets65 are used as guides for dicing the imaging structure 135 into aplurality of separate imaging structures 150. Then bond wires 145 arebonded to bond pads 115.

The bump bonding fabrication technique for back-illuminated imagers ofthe present invention provides a distinct advantage overfront-illuminated imagers manufactured by other techniques. FIG. 16shows a top plan view of a front illuminated imager 155. Since the frontilluminated imager 155 needs to have an unobstructed imaging area 175 toallow incident light to fall on the imaging pixels (not shown) in theimaging area 175, the bond pads 160 are distributed about the outerperiphery 165 of the top 170 of the front illuminated imager 155. It isdesirable to reduce the size of the front illuminated imager 155, whichrequires a reduction of the size of the pixels in the imaging area 175.Since the size of the bond pads 160 cannot be reduced, the ratio of thearea occupied by the bond pads 160 to the imaging area 175 increases.

FIG. 17A shows a top plan view of a back illuminated imager 180constructed in accordance with the bump-bonding technique of the presentinvention, while FIG. 17B is a side view. A plurality of bond pads 185can be distributed over the imaging area 190 and aligned with the vias195 of the support wafer 200. Since light enters only through the backside 205 of the imager 180, the bond pads 185 and vias 195 can be placedwithin the area 210 of the front side 215 of the imager 180 withoutaffecting the collection of light upon the pixels (not shown). In thisway, the overall size of the imager 180 can be reduced. The diced chipscan be packaged using conventional ball grade array or bump bondingtechnique.

It is to be understood that the exemplary embodiments are merelyillustrative of the invention and that many variations of theabove-described embodiments may be devised by one skilled in the artwithout departing from the scope of the invention. It is thereforeintended that all such variations be included within the scope of thefollowing claims and their equivalents.

1. A device, comprising: an insulator layer having a back side and afront side; an epitaxial layer substantially overlying the front side ofthe insulator layer; a dielectric layer formed substantially overlyingsaid epitaxial layer, the dielectric layer being distinct from saidinsulator layer; a plurality of bond pads substantially overlying saiddielectric layer; a handle wafer substantially overlying said dielectriclayer and having conductive trenches formed therein, said conductivetrenches being bonded to said plurality of bond pads; and at least oneimaging component at least partially underlying and extending into theepitaxial layer, wherein the at least one imaging component isconfigured to receive light through the back side of the insulatorlayer.
 2. The device of claim 1, further comprising a plurality ofalignment keys formed at least partially overlying and extending intosaid epitaxial layer.
 3. The device of claim 2, wherein said alignmentkeys are filled with one of an oxide of silicon, silicon carbide,silicon nitride, and poly-silicon.
 4. The device of claim 1, furthercomprising a plurality of bond pads substantially underlying andsubstantially overlying said conductive trenches.
 5. The device of claim1, further comprising an adhesive material that fills in gaps betweensaid handle wafer and said dielectric layer.
 6. The device of claim 1,wherein said conductive trenches formed in said handle wafer are made ofa metal.
 7. The device of claim 6, wherein said metal is one of gold,tin, and wolfram.
 8. The device of claim 1, further comprising at leastone optical component fabricated substantially overlying said epitaxiallayer and proximal to said insulating layer.
 9. The device of claim 8,wherein the said at least one optical component includes color filtersand micro-lenses, in any combination.
 10. The device of claim 9, furthercomprising a plurality of metallic contacts bonded to said conductivetrenches of said handle wafer.
 11. The device of claim 1, wherein saidhandle wafer is fabricated from one of silicon, aluminum nitride, andceramic.
 12. The device of claim 1, wherein said epitaxial layercomprises silicon and said insulator layer comprises an oxide ofsilicon.
 13. The device of claim 1, wherein said at least one imagingcomponent has an imaging area, and wherein at least one of saidplurality of bond pads substantially overlying said epitaxial layer alsoat least partially overlies said imaging area.
 14. The device of claim1, wherein the at least one imaging component includes at least one of aCMOS imaging component, a charge-coupled device (CCD) component, aphotodiode, an avalanche photodiode, and a phototransistor.